This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7119 from my application DISPERSION CONTROL FIBER AND A LARGE SIZE PREFORM MANUFACTURE METHOD THEREOF filed with the Korean Industrial Property Office on Jul. 22, 1999 and there duly assigned Serial No. 29828/1999.
1. Field of the Invention
The present invention relates to an optical fiber and a manufacturing method thereof, and more particularly to a dispersion control fiber and a method of manufacturing a large size preform thereof.
2. Description of the Related Art
Optical fibers as widely used as new media for transmitting information are largely classified into a single mode fiber and a multi mode fiber, according to the transmitting mode. The single mode fiber is also classified into a single mode fiber and a dispersion control fiber.
The dispersion control fiber comprises a dispersion shifted fiber with a zero dispersion wavelength band shifted to a wavelength band of 1.5 xcexcm, a dispersion flattened fiber with a constant dispersion value in a wide wavelength band of 1.3-1.6 xcexcm, and a non-zero dispersion shifted fiber with a low dispersion value in a wavelength band of 1.5-1.6 xcexcm. The dispersion shifted fiber is disclosed in U.S. Pat. No. 5,721,800 issued to Kato, et al., and the dispersion flattened fiber is disclosed in U.S. Pat. No. 5,675,690 issued to Nouchi, et al.
While the optical fibers have been made directly from raw materials, most of them generally have been made from a separate preform heated above a softening point within a furnace.
The length of an optical fiber capable of being drawn from the preform is dependent upon the diameter of the preform. In particular, since the drawing amount of the optical fiber is proportional to the preform diameter, methods of manufacturing a large size preform have been proposed. The size enlargement of preforms can be achieved by enlarging the diameter of a first preform formed by the deposition and collapse, and by enlarging the diameter of a tube for over-cladding.
FIG. 1 is a flow chart showing the process of manufacturing a general optical fiber. As shown in FIG. 1, the general method of manufacturing the optical fiber comprises steps of forming a preform (step 10), drawing the optical fiber (step 20), coating a sheath around the outer periphery of the optical fiber (step 30), and winding the optical fiber (step 40). Generally, the steps of drawing and coating are continually performed within a fiber drawing apparatus.
The step 10 is a process of forming a base preform to draw the optical fiber. The method of forming the preform comprises a vapor-phase axial deposition (VAD) method, an outer chemical vapor-phase deposition (OCVD) method, a plasma chemical vapor-phase deposition (PCVD) method, and a modified chemical vapor-phase deposition (MCVD) method, the MCVD method being widely used.
The process of manufacturing the preform by the MCVD method will hereinafter be explained in detail. Gas such as SiCl4 or GeCl4 is introduced in a deposition tube rotated at a constant speed, and a burner movable left and right heats the outer periphery of the tube. Particles are deposited on the inner surface of the deposition tube. The deposited particles are sintered, collapsed and closed by the heat of the burner to form a first preform with a core layer and a cladding layer. The first preform is treated by over-cladding to form a resultant preform.
The refractive difference between the core and cladding can be selected by adjusting components of gas supplied into the deposition tube, and the process of manufacturing the preform by using the MCVD method is disclosed in U.S. Pat. Nos. 4,389,230 and 5,397,372 in detail.
The drawing and coating steps 20 and 30 are continually performed in the optical fiber apparatus provided with a furnace and a coating machine to draw the optical fiber from the preform. When the preform is heated above a softening point in the furnace, the optical fiber is drawn through a drawing hole provided on the lower end of the furnace. Then, the optical fiber is coated by passing through the coating machine, and cooled by passing through a cooling machine.
In the winding step 40, the optical fiber is applied with a stress by a capstan, and is wound around a spool.
FIG. 2 is a cross sectional view illustrating the structure of a general single mode fiber, and FIG. 3 is a cross sectional view illustrating the structure of a conventional dispersion control fiber. The shown optical fibers mainly consists of SiO2.
As shown in FIGS. 2 and 3, the general single mode fiber 100 and the conventional dispersion control fiber 200 comprise a core 110 or 210, a cladding 120 or 220, and a sheath 130 or 230, respectively.
SiO2 is a main component of the core 110 or 210, and GeO2 is added to adjust the refractive index distribution. The cladding 120 or 220 comprises GeO2, P2O5 and Freon to adjust the refractive index distribution or reduce the deposition temperature, in addition to SiO2. While the claddings 120 and 220 are shown in a single layer to be easily understood, a multi-layered cladding formed by over-cladding may be adopted.
Generally, the single mode fiber 100 has a core diameter of 8-12 xcexcm and a relative refractive index of 0.35, and the dispersion control fiber 200 has a core diameter of 5-8 xcexcm and a relative refractive index of 0.7-0.15. In other words, the core 210 of the dispersion control fiber 200 has a diameter smaller than that of the general single mode fiber 100, but has a refractive index higher than that of the single mode fiber. The relative refractive index is expressed by (n12xe2x88x92n22)/(2n12)*100, wherein n1 is a maximum refractive index of the core, and n2 is a minimum refractive index.
The sheaths 130 and 230 function as inner protective layers for preventing the cores 110 and 210 and the claddings 120 and 220 from mechanical or chemical damage. The sheaths 130 and 230 are made of a plastic material such as a thermosetting resin.
Table 1 shows a variation of the optical characteristics of the general single mode fiber and the prior dispersion control fiber. In Table 2, the variation of the optical characteristics and the deformation of the deposition tube are indicated in dependence on the diameter increment of the preform and the drawing temperature. In Table 1, S represents SiO2, G represents GeO2, P represents P2O5, and F represents Freon.
As can be seen from the Examples 1 and 2, even though, in the case of a general single mode fiber, it is drawn at an increased drawing temperature, there is little or no variation in the photo characteristics, such as the zero dispersion wavelength, the mode field diameter, and the dispersion slope, insofar as the drawing of the fiber is carried out under a condition in which the preform used has a size enlarged from 50 mm to 66 mm, as compared to the case in which a preform involving no size enlargement is used.
However, as seen from Examples 3 and 4, even though, in the case of a dispersion control fiber, it is drawn using a preform which varies in diameter from 50 mm to 66 mm, there is significant variation in the photo characteristics, such as the zero dispersion wavelength, the mode field diameter, and the dispersion slope, as compared to the case in which a preform involving no size enlargement is used. This is because the core and clad of the dispersion control fiber are made of different compositions, and because the dispersion control fiber has a relatively small core diameter, as compared to that of the single mode fiber.
That is, the conventional dispersion control fiber comprises a core composed of SiO2 and GeO2, and a cladding composed of SiO2, GeO2, P2O5, and Freon, and the diameter of the core is smaller than that of the single mode fiber. Accordingly, when such a dispersion control fiber is drawn at an increased drawing temperature using a preform enlarged in diameter, stress may be generated due to a non-uniform temperature distribution between the core and the cladding. Furthermore, the refractive index distribution in the core may vary due to a viscosity difference between the core and the cladding.
The varying refractive index distribution in the core, as will be seen from Example 3 and 4, induces a variation in the optical characteristics of the dispersion control fiber. For example, the zero dispersion wavelength is varied in the range of 20-40 nm, the mode field diameter is varied in the range of 0.2-0.5 xcexcm, and the dispersion slope is varied in the range of 0.004-0.009 ps/nm2, relative to the dispersion control fiber with the preform having a smaller diameter.
It is an object of the present invention to solve the problems involved in the prior art, and to provide a dispersion control fiber and a method of manufacturing a large size preform.
In order to achieve the above object, according to one aspect of the present invention, there is provided a dispersion control fiber comprising a core composed of SiO2, GeO2, and P2O5, and a cladding composed of SiO2, GeO2, P2O5, and Freon.
According to another aspect of the present invention, there is provided a dispersion control fiber comprising a core composed of SiO2, GeO2, P2O5, and Freon, and a cladding composed of SiO2, GeO2, P2O5, and Freon.
According to still another aspect of the present invention, there is provided a dispersion control fiber comprising a core composed of SiO2, GeO2, and P2O5, a first cladding composed of SiO2, GeO2, P2O5, and Freon, and disposed around an outer periphery of the core, and a second cladding composed of SiO2 only, and disposed around an outer periphery of the first cladding.
According to still another aspect of the present invention, there is provided a method of manufacturing a large size preform for a dispersion control fiber by an MCVD process, the method comprising steps of depositing SiO2, GeO2, P2O5, and Freon in an inner periphery of a deposition tube to form a cladding layer, and depositing SiO2, GeO2, and P2O5 on an inner periphery of the cladding layer to form a core layer.